The Response D2's are effectively a standmount version of the old Response D15, which were a great speaker, and obviously brings the benefits of standmounts to the table. For me, ProAc speakers start with the Response series, and the D2's keep up ProAcs reputation for making great speakers.
The SCM19's are a bigger version of the 11's, which we think is are the star model of the range. For the money, these models punch way above their weight, and should be on everyone's shortlist.
You'll find the ATC's to be fairly neutral, and the ProAc's to sound a little sweeter - because of this the ProAc's will sound a little livelier at lower volumes. Both models work well against the wall (the ATC's are sealed, and the ProAc's are front ported), so are fairly room friendly. The ProAc's will be a slightly easier to drive due to their 88.5dB sensitivity (compared to the SCM19's 85dB). In the end it may come down to your amplification as to which you would go for - system synergy is extremely important at this quality level.